1. Field of the Invention
The present invention relates to a sheet post-processing apparatus and an image forming apparatus. More particularly, the invention relates to a sheet post-processing apparatus to be provided in an image forming apparatus forming an image on a sheet, such as a copying machine, a facsimile machine or a printer.
2. Description of Related Art
In the area of image forming apparatuses such as a copying machine, a type is recently proposed, in which a plurality of sheets bearing an image formed sequentially by image forming means are subjected to a prescribed post-processing and discharged onto a stack tray, incorporating a sheet post-processing apparatus known as a finisher.
Post-processing applied in the sheet post-processing apparatus (hereinafter simply referred to as a “finisher”) includes, for example, a processing called sorting or offset in which sheets are discharged at positions different between jobs, a staple processing of aligning ends of sheets for each job, and side-marking the ends of the aligned sheets, and a punching processing of punching a hole at ends of the aligned sheets. In the finisher, bundles of the thus post-processed sheets (hereinafter referred to as “sheet bundles”) are discharged onto a stack tray, thereby providing convenience to users when an image forming apparatus is shared by a plurality of users through network connection.
A known conventional finisher comprises an intermediate processing tray serving as a sheet stacking base which temporarily stacks sheets for carrying out the above-mentioned sorting or offsetting post-processing, conveying means such as a roller which conveys the sheet having an image formed thereon to the intermediate processing tray, aligning means which performs width-direction alignment for sheets on the intermediate processing tray, and sheet discharging means which discharges the sheet on the intermediate processing tray aligned by the aligning means onto the stack tray. The finisher based on the staple processing or the punching processing is provided with binding means called a stapler for binding the end of the sheet bundle on the intermediate processing tray or with punching means for punching a hole at the end of the sheet bundle on the intermediate processing tray.
In an image forming apparatus having a finisher added thereto, when an image is formed on the sheet in the image forming section, the sheet is mounted on the intermediate processing tray by the conveying means, and width-direction alignment is carried out by the aligning means on the intermediate processing tray. The sheet is then discharged by the sheet discharging means such as a discharging belt onto the stack tray.
When a selection is made to conduct offsetting the discharged sheet bundle for each job, in the conventional finisher, the aligning means is controlled so that the discharging position of the sheet bundle in the preceding job and the discharging position of the sheet bundle in the current job are different (shift) from each other. For example, the aligning means is controlled for each sheet stacked onto the intermediate processing tray so that the sheet is at a position on the near side relative to the apparatus main body in the preceding job, and the aligning means is controlled in the next job so that the sheet is positioned to the far side relative to the apparatus main body. In the following job, the aligning means is controlled so that the sheet is positioned on the near side.
However, in the conventional image forming apparatus as described above, when offsetting of the sheet bundle is selected upon execution of a new job, after the end of a prescribed job, from the state in which the sheet bundle has been removed from the stack tray, if the sheet bundle has been aligned to the near side in the immediately preceding job, the sheet bundle in the new job is started from the far-side alignment. When taking out the sheet bundle mounted on the stack tray in the offset state, therefore, it may become difficult to take out the lowermost sheet bundle, or the lowermost sheet is often left behind in some cases.